Possible problems with hips/lumbar vertebrae?
The recent timeline is, my 11yo Paint mare had a single joint bumping her flat palate mouth for about six months, then changed to double joint appropriately, (both O-ring snaffles, double is almost twice as thick as first), and the main form of riding was English w/t/c/g trail with some small jumps, though six months prior she had been in the middle of Show Jumper training and reached a height of 3ft 6in consistently as a bounce stride set up, couldn't do a real course yet. She's a cross between racing types and a few halter types, not too leggy, rather rectangular-built.
A few weeks ago, I put her on a lunge 3-4 times for the week in place of riding, and she displayed sore hocks and a big problem with the left hand lead after using the right hand lead; she could use the left lead okay, more comfortable on the right though, if I started her on the left first, but if I started her on the right first and then went left in simple lead changes, she would cross fire or pick up the right hand lead. I talked to my vet over the phone, and she told me to quit lunging altogether and ride without pushing her as far as anything like a sprint, so lots of walk with minimal canter. I noticed she wasn't sprinting in pasture as usual and favored to stand in her stall instead of exploring her paddock or pasture, so I locked her in the largest pasture nearest to a lot of neighbor horses where she would hopefully be persuaded to move around on her own. That was for five days, and now she put on a few pounds from the grass and is tracking up on both sides (wasn't tracking up regularly on the left side before), I gave her some bute and added on a fleece back pad for her, and yesterday she moved a lot better, offering me the left lead as easy as the right as the hour-and-a-half session went on. The sessions usually consist of 10 minute walk, then 20 minute trot, 10 minute walk, 3-5 minute canter, 5 minute walk, 15 minute trot, 5 minute canter, and the rest a walk with a long and low outline, lots of stretch, some practice with moving laterally. Because the bute helped even though it was half a dose appropriate for her, I'm thinking she has a real problem in her hind end that isn't just soreness from lunging. Before I lunged her though, I noticed her left hip was difficult to move on and off saddle, and she would be rather reluctant to pick up the left lead, so I think the lunging exaggerated a pre-existing problem.
I'll be calling my vet again to schedule an appt. to determine soundness and see if I need a chiropractor for her.
Until then, any ideas as to what medical problems she could have? What problems do horses usually get in their pelvis, lower back, and hock areas?